You don’t have to look far to find a freshstudy suggesting that e-cigarette use isn’t harmless, even if the products are likely less toxic than traditional tobacco cigarettes. But what about people sitting in the same room or living in the same home as someone who vapes? Is there such a thing as secondhand vape? And just how dangerous could it be?

In the simplest terms, yes, there are absolutely chemicals you can inhale from someone else’s e-cigarette. These aren’t released into the air by the device, like what happens when you light up a tobacco cigarette, but they are exhaled back out by the user.

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But many of us don’t see secondhand vaping as a major problem: A 2017 study published by the CDC found that around 40 percent of people thought secondhand vaping only caused little to some harm, while one third of people weren’t sure.

To be fair, vaping research has largely focused on what happens to the lungs, airways, and immune systems of people directly exposed to e-cigarettes—meaning the users themselves. So we’re still largely in the dark about the effects of secondhand vaping. This lack of knowledge doesn’t mean that being exposed to someone else’s vape cloud is perfectly safe though, according to Panagis Galiatsatos, an internal medicine physician and director of the tobacco treatment clinic at Johns Hopkins Medicine in Maryland.

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“If someone has asthma or other underlying pulmonary diseases, like emphysema, breathing in air that’s not air—that’s filled with these toxins, or irritants—it’s going to set them off.”

“We do know that many of the chemicals present in e-cigarettes are toxic. There are things like formaldehyde that have been shown to impact lung health or that can be carcinogenic. So we know they exist in e-cigarettes, and they will likely still exist in the ‘secondhand smoke’ produced by e-cigarettes,” Galiatsatos told Gizmodo by phone.

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A major question about e-cigarettes is whether they’re overall less harmful than conventional tobacco products. So far, the evidence is pretty strong that they are, even in the case of secondhand exposure. A report by the National Academies of Sciences and Medicine in 2018, for instance, found “moderate evidence that second-hand exposure to nicotine and particulates is lower from e-cigarettes compared with combustible tobacco cigarettes.”

Scott Weaver, an epidemiologist at Georgia State University’s School of Public Health who has studied the usefulness of e-cigarettes as a tool for quitting smoking, told Gizmodo in an email that “there remains the potential for negative health effects for secondhand exposure to e-cigarette aerosol, particularly for vulnerable populations and those with sustained, high exposure.”

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According to Sven-Eric Jordt, an anesthesiologist, pharmacologist, and cancer biologist at Duke University who has studied the effects of aerosolized flavoring chemicals in these products, e-cigs could be also be spewing out other chemicals into the air that cigarettes don’t. And we don’t really know what they could do to us.

“In general, indoor pollution due to e-cigarettes is lower than by cigarettes, but, as for inhalation, e-cigarettes produce compounds that combustible cigarettes don’t, with unknown health effects,” Jordt told Gizmodo by email.

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Some recent research has looked specifically into secondhand vaping. A study published just this April, Jordt noted, found evidence that vaping can release substances like copper, propylene glycol, and tobacco-related carcinogens into the air. The study also estimated that in small spaces with poor ventilation, like in a car with the windows closed, these chemicals could have an acute effect on bystanders, such as by irritating their lungs. Another study in March found that enough of these chemicals can accumulate in places like vape shops to leave a lingering residue on surfaces.

Right now, we’re missing a concrete link between secondhand vaping and health risks. But that was also the case for a long time with first- and secondhand smoke from traditional cigarettes, Galiatsatos noted. It took lots of evidence, from different areas of research (animal studies as well as long-term observational studies in people) for scientists to definitively declare tobacco products a major cause of diseases like lung cancer.

It will take some time to accumulate this same sort of data for e-cigarettes. But many tobacco control experts believe that e-cigarettes should already be as heavily regulated as other tobacco products, given how common they’re becoming among teens who might have never smoked otherwise. And these regulations do include bans or restrictions on their public use.

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“With combustible cigarettes, I have plenty of information to tell my patients so they can make an informed, insightful choice. If my patients choose to smoke, I’m not going to fault them. It’s something that’s legal, they’re human beings—there’s no judgement on my part,” Galiatsatos said. “The challenge with e-cigarettes is that I don’t have that data to show them, so that they can make that informed decision.”

In the meantime, Galiatsatos has no problem with people—especially if their lungs are already sensitive to environmental triggers—asking vapers to abstain while in close quarters.

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“If someone has asthma or other underlying pulmonary diseases, like emphysema, breathing in air that’s not air—that’s filled with these toxins, or irritants—it’s going to set them off,” Galiatsatos said. “I have plenty of patients who, anecdotally, have come to me and said, ‘Oh, my roommate uses e-cigarettes, and every time I’m around them, I start wheezing a lot more.’”